The HER protein family consists of 4 members HER1, also named epidermal growth factor receptor (EGFR) or ErbB-1, HER2, also named ErbB-2, ErbB-3, also named HER3 and ErbB-4, also named HER4. The ErbB family proteins are receptor tyrosine kinases and represent important mediators of cell growth, differentiation and survival. The HER family represent receptors proteins of different ligands like the neuregulin (NRG) family, amphiregulin, EGF and (TGF-a). Heregulin (also called HRG or neuregulin NRG-1) is e.g. a ligand for HER3 and HER4.
Human HER3 (ErbB-3, ERBB3, c-erbB-3, c-erbB3, receptor tyrosine-protein kinase erbB-3, SEQ ID NO: 3) encodes a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases which also includes HER1 (also known as EGFR), HER2, and HER4 (Kraus, M. H. et al, PNAS 86 (1989) 9193-9197; Plowman, G. D. et al, PNAS 87 (1990) 4905-4909; Kraus, M. H. et al, PNAS 90 (1993) 2900-2904). Like the prototypical epidermal growth factor receptor, the transmembrane receptor HER3 consists of an extracellular ligand-binding domain (ECD), a dimerization domain within the ECD, a transmembrane domain, an intracellular protein tyrosine kinase domain (TKD) and a C-terminal phosphorylation domain. This membrane-bound protein has a Heregulin (HRG) binding domain within the extracellular domain but not an active kinase domain. It therefore can bind this ligand but not convey the signal into the cell through protein phosphorylation. However, it does form heterodimers with other HER family members which do have kinase activity. Heterodimerization leads to the activation of the receptor-mediated signaling pathway and transphosphorylation of its intracellular domain. Dimer formation between HER family members expands the signaling potential of HER3 and is a means not only for signal diversification but also signal amplification. For example the HER2/HER3 heterodimer induces one of the most important mitogenic signals via the PI3K and AKT pathway among HER family members (Sliwkowski M. X., et al, J. Biol. Chem. 269 (1994) 14661-14665; Alimandi M, et al, Oncogene. 10 (1995) 1813-1821; Hellyer, N. J., J. Biol. Chem. 276 (2001) 42153-4261; Singer, E., J. Biol. Chem. 276 (2001) 44266-44274; Schaefer, K. L., Neoplasia 8 (2006) 613-622) For an overview of HER3 and its various interactions within the HER receptor family and the NGR ligands family see e.g. G Sithanandam et al Cancer Gene Therapy (2008) 15, 413-448.
Amplification of this gene and/or overexpression of its protein have been reported in numerous cancers, including prostate, bladder, and breast tumors. Alternate transcriptional splice variants encoding different isoforms have been characterized. One isoform lacks the intermembrane region and is secreted outside the cell. This form acts to modulate the activity of the membrane-bound form. Additional splice variants have also been reported, but they have not been thoroughly characterized.
Interestingly in its equilibrium state, the HER3 receptor exists in its “closed confirmation”, which does mean, the heterodimerization HER3beta-hairpin motive is tethered via non-covalent interactions to the HER3ECD domain IV (see FIGS. 1c and 1d). It is supposed, that the “closed” HER3 conformation can be opened via the binding of the ligand heregulin at a specific HER3 heregulin binding site. This takes place at the HER3 interface formed by the HER3 ECD domains I and domain III. By this interaction it is believed, that the HER3 receptor is activated and transferred into its “open conformation” (see FIGS. 1e and 1b and e.g. Baselga, J. et al, Nat Rev Cancer 9 (2009). 463-475 and Desbois-Mouthon, C., at al, Gastroenterol Clin Biol 34 (2010) 255-259). In this open conformation heterodimerization and transignal induction with HER2 is possible (see FIG. 1b)
WO 2003/013602 relates to inhibitors of HER activity, including HER antibodies. WO 2007/077028 and WO 2008/100624 also relate to HER3 antibodies.
WO 97/35885 and WO2010/127181 relate to HER3 antibodies.
Human HER4 (also known as ErbB-4 ERBB4, v-erb-a erythroblastic leukemia viral oncogene homolog 4, p180erbB4 avian erythroblastic leukemia viral (v-erb-b2) oncogene homolog 4; SEQ ID NO:5) is a single-pass type I transmembrane protein with multiple furin-like cysteine rich domains, a tyrosine kinase domain, a phosphotidylinositol-3 kinase binding site and a PDZ domain binding motif (Plowman G D, wt al, PNAS 90:1746-50 (1993); Zimonjic D B, et al, Oncogene 10:1235-7 (1995); Culouscou J M, et al, J. Biol. Chem. 268:18407-10 (1993)). The protein binds to and is activated by neuregulins-2 and -3, heparin-binding EGF-like growth factor and betacellulin. Ligand binding induces a variety of cellular responses including mitogenesis and differentiation. Multiple proteolytic events allow for the release of a cytoplasmic fragment and an extracellular fragment. Mutations in this gene have been associated with cancer. Alternatively spliced variants which encode different protein isoforms have been described; however, not all variants have been fully characterized.
Anti-HER4 antibodies for use in anti-cancer therapy are known e.g. from U.S. Pat. No. 5,811,098, U.S. Pat. No. 7,332,579 or Hollmén M, et al, Oncogene. 28 (2009) 1309-19 (anti-ErbB-4 antibody mAb 1479).
So far it was not possible to select antigen binding proteins like e.g. antibodies that specifically bind to the beta-hairpin of HER3 and/or HER4 as these beta-hairpins of HER3 or of HER4 both represent hidden epitopes, which are not accessible in the equilibrium state of these receptors (see FIG. 1).